The present invention generally relates to the field of electro-acoustical drivers for inkjet printing heads, and in particular, to a method for eliminating reflecting waves in a piezo crystal inkjet driver to optimize print performance.
Piezo crystals are used to drive drop-on-demand, non-thermal, inkjet nozzles. When the electrical pulse that is applied to a piezo crystal has a very short rise-time, the pressure pulse that is generated by a vibrating diaphragm acting on the ink is dominated by its acoustic propagation. A long pulse would compress the ink fluid slowly and acoustic propagation effects would be less significant. Piezo inkjet printing heads may use fast rise-time and high amplitude electrical drive pulses to launch large pressure wave pulses in order to eject fast ink drops. An inkjet structure about 1 millimeter would display an acoustic wave propagation delay of about 1 microsecond. Since 1 microsecond rise-time pulses are commonly used, acoustic delay is usually important.
The total acoustic energy in an ink chamber may be 100 times greater than the energy required to release the ink drop. The excess energy is a source of uncontrolled release of satellite drops, or interference with subsequent drops, or cross talk between nozzles, which could deteriorate printing head performance. Therefore, some mechanism for the dissipation of this acoustic energy is required.
Accordingly, it is a principal object of the present invention to overcome the disadvantages in the operation of prior art inkjet printer heads, and provide a matched pulse method to optimize inkjet print head performance by eliminating undesirable reflected waves from a piezo crystal driver.
It is another object of the invention to generate matching motions in an electro-acoustical diaphragm to the reflected waves returned from a nozzle surface that will act to neutralize or dampen the returning waves.
It is yet another object of the present invention to improve the rapidity of inkjet printing in the digital printer industry.
It is still another object of the present invention to use long inkjet acoustic channels and wide spacing between inkjet channels to reduce the prior art problems, including cross-talk and heating by the active piezo element.
In accordance with a preferred embodiment of the present invention there is provided a method for eliminating reflected waves from a vibrating diaphragm surface within an acoustic chamber of an inkjet printer piezo-crystal driver. The first step involves generating an electro-acoustical driven pressure waveform from the piezo-crystal driver, the diaphragm initially acting on ink to release it, wherein the waveform is reflected from a nozzle plate disposed remotely from the diaphragm. Further steps include determining an instant when the reflected pressure waveform returns to the diaphragm surface and producing a matched pulse waveform causing a reverse motion in the diaphragm exactly at that instant, whereby the reverse motion reverses the initial diaphragm action, such that the matched pulse waveform absorbs and eliminates the reflected pressure waveform at the exact instant.
Greater distance between the diaphragm, with its piezo driver, and the inkjet nozzle, forces the system to operate in an acoustic mode, where propagation delay and acoustic wave reflections are important. The matched pulse technique allows the use of short pulses and high drop ejection frequencies in a large inkjet chamber. The inkjet head based on the design discussed in this presentation operates as a drop-on demand inkjet head, at frequencies in the range of 100 kHz, but is not limited to that frequency.
Some printing heads are designed with labyrinth channels to dampen excess acoustic energy. A more efficient method is to use a matched pulse that reverses the diaphragm motion exactly at the instant that the reflected pressure wave returns to the diaphragm surface. In a simple one-dimensional structure, a matched pulse would have equal rise and fall rates separated by a time equal to the round trip acoustic delay. In general almost all of the acoustic wave energy that arrives at the nozzle plate is reflected back to its source, the piezo-driven deflection plate. If every increment of return motion of the deflection plate is the reverse of the corresponding increment of transmitting motion occurring earlier, then the reflected wave is entirely absorbed by the deflection plate.
The main advantage of the invention is the generation of a matched pulse to exactly cancel a reflected wave. The pulse features, including rise and fall rates, and pulse duration, are each matched to the reflected wave.
Thus, interference by satellite ink drops is eliminated, and cross-talk between print nozzles is minimized. Extraneous acoustic energy, in general, is dissipated.